Can you end Jaheira's Harper Quest without romancing her?
After Dermin attacked me, I waited for about 350 days (using ctrl + t) and still nothing happened. Ain't I supposed to meet Elminster or something?

Yes you can!
You must be in an outdoors area to receive the visit from elminster.Try going to the druid grove or to the umar hills.

Nope, I was outside. And I switched areas a couple of times. Bumping

I am at loss! I'll try and experiment with the console and let you know if I find any way to solve this

Jaheira's romance must be the buggiest thing in the whole damn game, and everything connected to her seems to suffer for it. I've never gotten further in the whole Harper deal than meeting Dermin once. (Can this subplot be kicked back into gear, or do I need to find the better version of Shadowkeeper?) I think her romance and Harper quest lock up if you start Chapter 4 before she gets called to the Harper Hold, as well. (At least, that's how it worked in my present game.)